1. Field of the Invention
This invention is concerned with improvements in or relating to the manufacture of moisture curable organosiloxane compositions.
2. Background Information
Moisture curable organosiloxane compositions are known. These compositions find use in various applications, for example as sealant compositions which can be applied to a joint between elements and cured to provide an elastomeric seal between them. These compositions cure at room temperature and are particularly attractive for use as sealant compositions for sealing, for example highway joints, joints in articles for example vehicle headlights and joints in buildings and in glazing applications, in view of the fact that no special heating or other cure conditions are generally required to produce a seal of desired quality.
Many moisture curable organosiloxane compositions have been proposed and are generally formed from an at least one substantially linear polyorganosiloxane containing at least two silanol groups, a crosslinker capable of reaction with the polyorganosiloxane to yield a crosslinked network, and catalyst materials. These compositions cure by a condensation reaction promoted by moisture.
The crosslinker in moisture curable organosiloxane compositions is generally selected from polyfunctional silanes which readily hydrolyze, those commonly employed being triacetoxy silanes, trialkoxy silanes, triamino silanes and trioximo silanes. It is believed that the condensation reaction proceeds via a capping of the polyorganosiloxane with, for example, dialkoxyalkylsilyl groups followed by interaction of the alkoxy groups of the end caps and or silanol groups to yield a crosslinked structure.
While some curing of the composition during manufacture and storage is acceptable, it is important that this curing does not proceed too far prior to application at its intended work site at which it is intended to cure under influence of atmospheric moisture. Thus it is important that exposure of the composition to moisture is kept to a uniform, acceptably low extent from batch to batch during manufacture and storage, otherwise the composition cures to an extent which renders it impractical for its intended purpose.
Moisture curable compositions based on organosilicon compounds generally contain finely divided fillers. The fillers generally used are those which strengthen the cured material, reduce the cost of the product or otherwise confer a desired combination of properties.
Typical fillers include but are not limited to high surface area silicas, ground quartz, iron oxide, zinc oxide, carbon black, calcium carbonate and diatomaceous earth.
Moisture curable organosiloxane compositions can be manufactured using a batch process during which the filler and polyorganosiloxane are mixed together, the crosslinker and catalyst are added to the mixture and the resultant composition is than packaged in containers such as cartridges, which are then sealed in an airtight manner to prevent ingress of moisture.
A continuous process for preparing moisture curable organosiloxane compositions using a devolatilizing extruder is described in U.S. Pat. No. 3,960,802, which issued to Beers et al. on Jul. 9, 1985. The ingredients of the composition comprise a silanol terminated polydiorganosiloxane, a crosslinking silane and a silanol-reactive organometallic ester compound of a metal, and an optional filler. This patent mentions a twin screw Werner-Pflieferer mixer Model 2SK as a preferred devolatilizing extruder.
U.S. Pat. No. 4,528,324, which issued to Chung et al. on Jul. 9, 1985 describes further developments in a continuous process for preparing moisture curable organosiloxane compositions using a devolatilizing extruder. Chung et al. teach using a catalyst for the end-capping reaction of the polydiorganosiloxane together with an optional scavenger to react with free silanol groups.
Recycling a major portion of a rubber composition from one section of an extruder through an annular chamber back to a previous section of the extruder to increase the time during which the composition is exposed to the masticating action of the of the screws in the extruder is taught in U.S. Pat. No. 4,897,236, which issued to Rabiger et al. on Jan. 30, 1990.
Among the various crosslinkers which have been proposed for moisture curable organosiloxane compositions the alkoxysilanes are particularly attractive because by-products of the condensation are neither acidic nor alkaline, so that generally they do not cause corrosion of substrates with which the curing material comes in contact. However, the alkoxysilanes yield alcohol as a by-product of the condensation reaction and while this is generally acceptable when the composition is used under well vented conditions, escape of the alcohol during manufacture of the composition is regarded as undesirable.
Attempts to prepare moisture curable organosiloxane using trialkoxysilanes as the crosslinker by mixing the ingredients in a twin screw extruder have revealed difficulties due to the need for the composition to remain resident in the extruder for sufficient time to permit that part of the cure in which end-capping of the polymer occurs to proceed to a desired extent and adequate venting of water and by-product from the composition prior to exit of the composition from the extruder. While these requirements may be met by use of an extruder of sufficient length or by linking two or more extruders in series, this is undesirable on a cost basis.